1. Field of the Invention
The present invention relates to a method and apparatus for feedback control of an air-fuel ratio in an internal combustion engine having two air-fuel ratio sensors upstream and downstream of a catalyst converter disposed within an exhaust gas passage.
2. Description of the Related Art
Generally, in a feedback control of the air-fuel ratio sensor (O.sub.2 sensor) system, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and detected engine speed, and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio. According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratios around the stoichiometric ratio required for three-way reducing and oxidizing catalysts (catalyst converter) which can remove three pollutants CO, HC and NO.sub.X simultaneously from the exhaust gas.
In the above-mentioned O.sub.2 sensor system where the O.sub.2 sensor is disposed at a location near the concentration portion of an exhaust manifold, i.e., upstream of the catalyst converter, the accuracy of the controlled air-fuel ratio is affected by individual differences in the characteristics of the parts of the engine, such as the O.sub.2 sensor, the fuel injection valves, the exhaust gas recirculation (EGR) valve, the valve lifters, individual changes due to the aging of these parts, environmental changes, and the like. That is, if the characteristics of the O.sub.2 sensor fluctuate, or if the uniformity of the exhaust gas fluctuates, the accuracy of the air-fuel ratio feedback correction amount FAF is also fluctuated, thereby causing fluctuations in the controlled air-fuel ratio.
To compensate for the fluctuation of the controlled air-fuel ratio, double O.sub.2 sensor systems have been suggested (see: U.S. Pat. Nos. 3,939,654, 4,027,477, 4,130,095, 4,235,204). In a double O.sub.2 sensor system, another O.sub.2 sensor is provided downstream of the catalyst converter, and thus an air-fuel ratio control operation is carried out by the downstream-side O.sub.2 sensor in addition to an air-fuel ratio control operation carried out by the upstream-side O.sub.2 sensor. In the double O.sub.2 sensor system, although the downstream-side O.sub.2 sensor has lower response speed characteristics when compared with the upstream-side O.sub.2 sensor, the downstream-side O.sub.2 sensor has an advantage in that the output fluctuation characteristics are small when compared with those of the upstream-side O.sub.2 sensor, for the following reasons.
(1) On the downstream side of the catalyst converter, the temperature of the exhaust gas is low, so that the downstream-side O.sub.2 sensor is not affected by a high temperature exhaust gas.
(2) On the downstream side of the catalyst converter, although various kinds of pollutants are trapped in the catalyst converter, these pollutants have little affect on the downstream-side O.sub.2 sensor.
(3) On the downstream side of the catalyst converter, the exhaust gas is mixed so that the concentration of oxygen in the exhaust gas is approximately in an equilibrium state.
Therefore, according to the double O.sub.2 sensor system, the fluctuation of the output of the upstream-side O.sub.2 sensor is compensated for by a feedback control using the output of the downstream-side O.sub.2 sensor. Actually, as illustrated in FIG. 1, in the worst case, the deterioration of the output characteristics of the O.sub.2 sensor in a single O.sub.2 sensor system directly effects a deterioration in the emission characteristics. On the other hand, in a double O.sub.2 sensor system, even when the output characteristics of the upstream-side O.sub.2 sensor are deteriorated, the emission characteristics are not deteriorated. That is, in a double O.sub.2 sensor system, even if only the output characteristics of the downstream-side O.sub.2 are stable, good emission characteristics are still obtained.
In the above-mentioned double O.sub.2 sensor system, however, an air-fuel ratio correction coefficient FAF2 or an air-fuel ratio feedback control parameter such as a skip amount RSR (RSL) controlled by the output of the downstream-side O.sub.2 sensor in an air-fuel ratio feedback control state may be greatly deviated from such an air-fuel ratio correction coefficient or an air-fuel ratio feedback control parameter in a non air-fuel ratio feedback control (open control) state. As a result, in this case, when the engine control is changed from an open control state to an air-fuel ratio feedback control state by the upstream-side and downstream-side O.sub.2 sensors, since the response speed of an air-fuel ratio feedback control operation by the downstream-side O.sub.2 sensor is smaller than that of the upstream-side O.sub.2 sensor, it will take a long time for the air-fuel ratio correction coefficient FAF2 or the skip amount RSR (RSL) to reach an optimum level, i.e., it will take a long time for the controlled air-fuel ratio to reach an optimum level, thereby causing an overrich or overlean condition in the controlled air-fuel ratio, and thus deteriorating the fuel consumption, the drivability, and the condition of the exhaust emissions such as HC, CO, and NO.sub.X, since the air-fuel ratio correction coefficient FAF2 (=0.1) or the skip amount RSR (RSL) during an open-loop control is, in this case, not an optimum level, which is a problem.
Also, even during an air-fuel ratio feedback control by the downstream-side O.sub.2 sensor, when the engine is transferred from one driving region to another driving region, the optimum level of the controlled air-fuel ratio is shifted, thus creating the above-mentioned problem.
Note, it has been disclosed in U.S. Ser. No. 903,977 that, in a double air-fuel ratio sensor system including two O.sub.2 sensors upstream and downstream of a catalyst converter provided in an exhaust passage, an actual air-fuel ratio is adjusted by using the output of the upstream-side O.sub.2 sensor and the output of the downstream-side O.sub.2 sensor. A center value of the air-fuel ratio correction coefficient FAF2 or the air-fuel ratio feedback control parameter such as the skip amount RSR (RSL) is calculated by a learning control for each driving region defined by a load parameter such as the intake air amount Q, and an air-fuel ratio feedback control is initiated by using such a center value for the current driving region when the engine enters into an air-fuel ratio feedback control state or when the engine is transferred from one driving region to another driving region.